What Does Chrome Do To a Gem?

[iLander]

I got this email from Barry about a potential stone for the bubblegum ring:


"about the red stone. I just cut it yesterday. It had a great deal of chromium in
it and looked like a chrome pyrope from northern AZ under the Chelsea filter. I'll have to ask my resident color
expert about the tone of the red. It is noticeably lighter than my other red garnets from Tanzania, but I don't know if
it would still be too dark for you."

It sounds good and I'm hopeful! But what does it mean?

Sorry, I know this is probably a dumb question but google was no help . . .

 

[chrono]

Chromium does different things for different gemstones. In emeralds (green beryl), chrome tourmaline, chrome sphene and the like, it gives the stone that awesome rich green colour. In rubies (red corundum), chromium imparts that rich vibrant red colouring. I’m not sure of chromium being the chromophore for those Ant Hill garnets; I’ll have to check my books.

 

[chrono]

You might find this an interesting read on how different trace minerals affect the colour of a particular gem variety:
http://www.emporia.edu/earthsci/amber/go340/visual.htm

A quotation of the reason why chromium causes red in one gemstone and green in another:
In allochromatic minerals how can one element be responsible for two different colors? Chromium is responsible for the green color of emerald, the red color of ruby, and the red and green color of alexandrite! Ruby is the red variety of the colorless mineral corundum, Al2O3; the red results when about 1% of the aluminum (Al3+) are replaced with chromium ions (Cr3+). The red color is created by the absorption of light by the chromium in the yellow-green and violet portions of the visible light spectrum, and transmission in the red and minor amounts of blue spectral colors. The absorption and transmission of certain wavelengths of light depends on the electron configuration in chromium, but also on the crystal structure the ions are embedded in. In emerald, 3BeOAl2O36SiO2, the chromium substitutes for the aluminum, but the beryllium and silicon oxides change the crystal structure and thus the frequencies of light that are absorbed. Emerald absorbs violet and yellow-red portions of the spectrum, transmitting the green-blue wavelengths. Alexandrite, or chrysoberyl, is a mineral with a chemical formula between that of corundum and beryl, BeOAl2O3. Chromium replaces aluminum ions, which interact with the beryllium oxide structure, resulting in both red and green colors. Candlelight has abundant yellow and red light, and when passed through the alexandrite the mineral absorbs the blue wavelengths present and transmits the red. Daylight contains more blue wavelengths, so that this mineral appears green-blue and absorbs the red light.

 

[iLander]

Hi Chrono,

Thank you Loving that article!

What would it do to Barry's garnet?

Dumb it down for me please . . . I'm not the sharpest tool in the shed

 

[chrono]

In general, pyrope garnets are coloured by trace amounts of iron and sometimes chromium. I believe that the higher the Cr+3 content, the redder the stone will appear.